Two forms of citrullinemia have been described, both having different signs and symptoms, and are caused by mutations in different genes. Citrullinemia belongs to a class of genetic diseases called urea cycle disorders. The urea cycle is a sequence of chemical reactions that takes place in the liver. These reactions process excess nitrogen, generated when protein is used by the body, to make a compound called urea that is excreted by the kidneys.
Type I citrullinemia is the most common form of the disorder, affecting about 1 in 57,000 births worldwide. Mutations in the ASS gene cause type I citrullinemia. The enzyme made by this gene, argininosuccinate synthetase is responsible for one step of the urea cycle. Mutations in the ASS gene reduce the activity of the enzyme, which disrupts the urea cycle and prevents the body from processing nitrogen effectively. Excess nitrogen, in the form of ammonia, and other byproducts of the urea cycle accumulate in the bloodstream, leading to the characteristic features of type I citrullinemia.
Type II citrullinemia may also develop in people who had a liver disorder called neonatal intrahepatic cholestasis during infancy. This condition blocks the flow of bile and prevents the body from processing certain nutrients properly. In many cases, the signs and symptoms resolve within a year. Years or even decades later, however, some of these people develop the characteristic features of adult type II citrullinemia.
Type II citrullinemia is primarily found in the Japanese population, where it occurs in an estimated 1 in 100,000 to 230,000 individuals. Type II has also been reported in people from East Asian and Middle Eastern populations. Mutations in the SLC25A13 gene are responsible for type II citrullinemia. This gene makes a protein called citrin, which normally shuttles certain molecules in and out of mitochondria. These molecules are essential for the urea cycle and are also involved in making proteins and nucleotides. Mutations in the SLC25A13 mutation typically prevent the production of any functional citrin, which inhibits the urea cycle and disrupts the production of proteins and nucleotides. The resulting buildup of ammonia and other toxic substances leads to the signs and symptoms of type II citrullinemia. Researchers have determined that many infants with neonatal intrahepatic cholestasis have the same mutations in the SLC25A13 gene as adults with type II citrullinemia.